I have a 1st gen Nexus 7 and I've been having trouble copying files to and from my Nexus 7.
The first problem is that when I plug my N7 into my Windows 7 PC, using the factory cable, the device shows up in My Computer, then immediately disconnects, then reconnects then disconnects and finally connects. This is so infuriating as for about 15 seconds it's just showing then disappearing constantly.
Once its' finally connected, I'll navigate to a file and drag it to my PC, to copy. As soon as that happens, an error box saying "A device attached to the system is not functioning". Then the explorer window disappears and my device has to reconnect.
It's driving me mad, it's not worked for about 6 months and I'm at the point of smashing it with a hammer. I've completely re-installed Android from Google's stock ROM. I've re-installed the drivers on my PC, got rid of the ADB drivers.